Field
Embodiments of the invention relates to semiconductor wafer processing systems and, more particularly, to a method and apparatus for removing residual charges on an electrostatic chuck used to retain a semiconductor wafer.
Description of the Related Art
In high precision manufacturing, for example semiconductor manufacturing, a substrate may need to be precisely held by a fixture during the manufacturing operations to increase uniform quality and reduce defects. In some manufacturing operations, an electrostatic chuck is used as the fixture to hold the substrate against a supporting structure with an electrostatic force (“clamping force”) during one or more manufacturing operations. Although electrostatic chucks vary in design, electrostatic chucks utilize the application of a voltage to one or more electrodes embedded in the chuck so as to induce opposite polarity charges in the substrate and the electrode(s), thus generating an electrostatic clamping force. The electrostatic clamping force pulls the substrate against the chuck, thereby retaining the substrate.
A problem with electrostatic chucks is the difficulty of removing the electric charge from the substrate and the chuck when it is desired to release the substrate from the chuck. One conventional solution is to connect both the electrode and the substrate to ground to allow the charge to drain. Another conventional solution is to reverse the polarity of the DC voltage applied to the electrodes.
A shortcoming of these approaches is that often a residual charge remains on the chuck resulting in some electrostatic force remaining between the substrate and the chuck. Additionally, the remaining electrical charge may accumulate regionally such that the electrostatic force applied to substrate may become uneven across the substrate. The residual electrostatic force may also necessitate the use of a large mechanical force to separate the workpiece from the chuck. Undesirably, the force required to remove the substrate often cracks or otherwise damages the substrate. Even when the substrate is not damaged, the difficulty of mechanically overcoming the residual electrostatic force sometimes causes the substrate to pop off the chuck unpredictably into a position from which it is difficult to retrieve using a substrate transport robot.
The accumulation of the residual charge is also detrimental to chucking of a subsequent substrate. The accumulated charge interferes with the chucking voltage by either being additive (i.e., the accumulated charge has the same polarity as the chucking voltage) or being subtractive (i.e., the accumulated charge has an opposite polarity as the chucking voltage).
In certain ceramic electrostatic chucks, high temperature exposure (e.g., exposure to temperatures greater than 200° C.) may make the material of the chuck more conductive. As such, some of the residual charge will dissipate by conduction through the chuck to the electrodes as long as the chuck is maintained at the high temperature. Additionally, a plasma cleaning step may be performed after wafer processing. Typically, an inert gas, such as argon, is introduced into the chamber containing the electrostatic chuck and ionized into a plasma. The plasma may form a conductive path from the surface of the electrostatic chuck to a grounded chamber component, such as a wall of the chamber. The ions from the plasma bombard the electrostatic chuck surface thereby dislodging the residual charges. The dislodged charges then move through the plasma to the ground. The plasma cleaning step may also include high temperatures to further enhance the dissipation of the residual charges. Unfortunately, the dissipating effect of the high temperature maintenance process (either by direct heating, or plasma processing) is greatly reduced as the chuck cools and is prepared for substrate processing.
Therefore, there is a need for an improved electrostatic chuck.